A PWM tranistor bridge inverter drive is disclosed in my copending U.S. application Ser. No. 558,497 filed Mar. 14, 1975, entitled Transistor Bridge Inverter Motor Drive Having Reduced Harmonics and having the same assignee as this invention, which controls frequency and magnitude of stator voltage applied to an induction motor so as to maintain constant flux in the motor over a wide speed range and to substantially reduce harmonics in the current supplied to the motor. The inverter drive disclosed in my copending application controls the voltage-to-frequency (volt/hertz) ratio so as to maintain constant flux in the motor and can be adjusted so that motor output torque is constant over the entire speed range or can alternatively be adjusted so that the motor provides substantially constant rated torque below motor base speed and rated horsepower above base speed. The inverter drive of my copending application integrates the output voltage of the bridge inverter with respect to time over a fixed fraction of a cycle and compares the integral to a fixed level to effect constant flux in the motor and also has adjustable means to selectively vary the volt/hertz ratio and thus regulate the slope of the motor terminal voltage versus frequency characteristic.
The inverter drive disclosed in my copending application maintains the volt/hertz ratio constant and thus should theoretically maintain the magnetic flux (whose magnitude is proportional to the rate of change of voltage) in the motor constant over the entire speed range. However, the magnetic flux in a motor decreases at low motor speeds even though the volt/hertz ratio remains constant. The effective resistance drop in a motor is substantially constant in magnitude but is so much lower than the leakage reactance drop at speeds above motor base speed that it can be considered negligible. However, at low motor speeds the resistance voltage drop across the copper approaches the magnitude of the reactance voltage drop across the magnetizing inductance of the stator winding, thereby decreasing the magnitude of magnetizing current (and the magnetic flux density which is aproximately proportional to ampere turns, neglecting saturation) at low frequencies.